Linear motors have been used in industrial devices such as machine tools in order to achieve high speed and high accuracy. In a linear motor, permanent magnets are provided in a movable member or a stator, such that a magnetic attractive force several times as strong as a thrust acts between the movable member and the stator. The magnetic attractive force may disadvantageously deform the machine tool to reduce processing accuracy. To solve this problem, a magnetic attractive force offsetting linear motor is used. See, for example, Japanese Patent Laid-Open No. 2005-137140 (Patent Document 1). The magnetic attractive force offsetting linear motor is composed of a movable member and two stators arranged parallel to each other so as to sandwich the movable member between the stators. Such a configuration allows a magnetic attractive force to be generated between each of the two stators and the movable member such that the generated magnetic attractive forces act in opposite directions so as to offset each other. This minimizes the overall magnetic attractive force, and helps preventing the magnetic attractive force from adversely affecting operation of the machine tool.
An example of a conventional linear motor will be described with reference to FIGS. 5 to 8. FIG. 5 is a diagram showing a general configuration of the conventional linear motor. FIG. 6 shows a sectional view taken along line C-C in FIG. 5. FIG. 7 is a diagram showing coils wound around the linear motor. FIG. 8 is a perspective view of a stator.
The linear motor has two stators 52a and 52b extending in parallel and a movable member 51 that is movable between the stators 52a and 52b along a direction in which the stators 52a and 52b extend.
Each of the stators 52a and 52b is formed of stacked electromagnetic steel plates. Each of the stators 52a and 52b has salient poles 50 arranged at a pitch P. As shown in FIG. 8, each of the stators 52a and 52b is produced to have a predetermined length. The plurality of pieces of each stator are arranged over a stroke length of the movable member 51 in a direction in which the movable member 51 moves. The stators 52a and 52b are fixed to, for example, a base 72 (shown in FIG. 6) of the machine tool. Specifically, as shown in FIGS. 6 and 8, each of the stators 52a and 52b is fixed to the base 72 by bolts 71 so that a bottom surface 74 of the stator contacts the base 72.
On the other hand, the movable member 51 is supported in such a manner that it can be moved, in the direction of an X axis in FIG. 5, by a rolling guide or the like provided between the base 72 and a table (not shown in the drawings) and fixed to the table. The movable member 51 is composed of movable member blocks 53, 54, and 55 each formed of stacked directional electromagnetic steel plates that exhibit an excellent magnetic characteristic in the direction of a Z axis that is perpendicular to the direction of the X axis, in which the movable member 51 advances. The movable member block 53 is for a U phase, the movable member block 54 is for a W phase, and the movable member block 55 is for a V phase. The movable member blocks 53, 54, and 55 are arranged such that each of the movable member blocks 53, 54, and 55 is displaced by 120° that is, by one-third of the magnetic pole pitch P of the stators 52a and 52b, relative to the direction of the X axis, in which the movable member 51 advances. A three-phase AC coil is wound around each of the movable member blocks 53, 54, and 55. That is, a three-phase AC coil 56 for the U phase is wound around the movable member block 53. A three-phase AC coil 57 for the W phase is wound around the movable member block 54. A three-phase AC coil 58 for the V phase is wound around the movable member block 55. The U, W, and V phases of the three-phase AC coils 56, 57, and 58 are connected together in the form of Y as shown in FIG. 7. The movable member blocks 53, 54, and 55 around which the three-phase AC coils 56, 57, and 58 are wound are integrated with one another with a mold resin 76
Permanent magnets 59 and 64 are arranged on a surface of each of the movable member blocks 53, 54, and 55 such that N poles and S poles alternate. Specifically, as shown in FIGS. 5B and 5C, three pairs each of an N-pole permanent magnet and an S-pole permanent magnet are arranged at the pitch P as shown in FIGS. 5B and 5C. Here, as shown in FIG. 5, when the stator 52a side is defined as SIDE-A and the stator 52b side is defined as SIDE-B, the permanent magnets 59 on the SIDE-A and the permanent magnets 64 on the SIDE-B are arranged such that the polarity on the SIDE-A is opposite that of the SIDE-B.
As described above, the three-phase AC coils 56, 57, and 58 are connected together in the form of Y. When a current is applied to the three-phase AC coils 56, 57, and 58 in the direction from U to V and W, a magnetic flux 62 is excited in the linear motor.
An example of operation of the linear motor will be described. When a current is applied to the three-phase AC coils 56, 57, and 58, the movable member blocks 53, 54, and 55 are excited in the plus or minus direction along the direction of the Y axis. As a result, the magnetic flux is strengthened in those of the permanent magnets 59 and 64 aligned in the same magnetization direction as the direction in which the AC coil is excited, while the magnetic flux is weakened in the remaining permanent magnets, those aligned in a magnetization direction opposite to the excitation direction. Thus, each of the permanent magnets 59 and 64 are excited to have either one of the opposite polarities, that is, to serve as either the N or S pole. The magnetic flux passing through the movable member block 53, 54, and 55 and the stators 52a and 52b side forms a magnetic path as shown at reference numeral 62 in FIG. 5A. At this point, a magnetic attractive force is generated depending on the positions of the movable member 51 and the stators 52a and 52b. Thus, a thrusting force is generated in the movable member 51, which is thus moved.
The flow of the magnetic flux will be described in further detail using an example in which a current is directed from the U phase to the V and W phases, that is, the current flows through the three-phase AC coil 56 in the winding direction shown in FIG. 5A and through the three-phase AC coils 57 and 58 in the direction opposite to the winding direction shown in FIG. 5A. Then, the SIDE-A of the movable member block 53 becomes the S pole, and the SIDE-B thereof becomes the N pole. In contrast, the SIDE-A of the movable member blocks 54 and 55 becomes the N pole, where the SIDE-B thereof becomes the S pole. Thus, as shown in FIG. 5A, the magnetic path 62 is formed such that the magnetic flux flows from the movable member block 53 through the stator 52b to the movable member blocks 54 and 55 and then returns through the stator 52a to the movable member block 53. Then, the magnetic attractive force acts on the movable member 51 in the direction of the X axis to generate a thrust in the movable member 51.